1. Field of the Invention
The present invention relates to an engine generator apparatus and particularly to an engine generator apparatus such as a generator facility for private use or a small-sized cogeneration system having a function for interconnecting with an electric power grid.
2. Description of the Related Art
Generator apparatuses for private use have widely been provided for emergency use in case of blackout or power failure. Recently, private use cogeneration type generator apparatuses which can be interconnected with electric power systems for improvement of the efficiency of operation are getting popular. Such a cogeneration type private use generator apparatus comprises a small generator driven by a gasoline engine or a gas engine fueled with gas fuel such as town gas.
For smooth interconnection or parallel operation of a electric power grid, the cogeneration type generator apparatus needs to comply with the guideline for technical requirements for interconnection (issued by the Ministry of Trade) which stipulates technical standards including a range of outputs of power systems and protections for the power grid in case of ground fault or short-circuit. It is hence necessary to constantly monitor the apparatus for inhibiting discrepant actions from the requirements of the guideline due to a fault in the operation of the generator. A grid is a power network being a systematic network of power lines.
The output of the generator described above is rectified with three phase windings and converted into an alternating current with a commercial frequency (the source frequency of a power system). A fault in the operation of the ordinary or relatively large output generator may be detected by constantly monitoring the output voltage of each of the three phase windings. However, when a small output generator apparatus such as a private use cogeneration system which is substantially low in the price is quipped with the same monitoring means as of the large output power generator, its overall price will increase thus resulting in the higher production cost.
The present invention is developed in view of the above drawback and its object is to provide an engine generator apparatus which can detect a fault in the operation of its generator section with a simple arrangement.
A first feature of this invention is that an engine generator apparatus for rectifying and converting an alternating output of a generator, which has multi-phase windings and is driven by an engine, and converting the rectified output by an inverter into an alternating current at the frequency of a power system and the alternating current is interconnected with the source of said power system, comprising, a means for starting the interconnection with the system source when a direct current voltage rectified rises up to first predetermined level after the start up of the engine and then increasing the output of the inverter, means for canceling the interconnection when the direct current voltage drops down to below second predetermined level, and for re-starting the interconnection with the system source when the direct current voltage returns back to the first predetermined level, and a fault detecting means for judging that the power generator has a fault when the direct current voltage drops down to below the second predetermined level after the re-starting of the interconnection.
According to the first feature of the present invention, when the direct current voltage rectified after the start up of the engine rises up to the predetermined level, the connection with the electric power system is made and the output of the inverter is increased. At the time, the direct current voltage drops down if a fault such as line breakage in the windings occurs. However, the direct current voltage rises up again upon cancellation of the interconnection. It can then be judged that the generator has a fault when the direct current voltage drops down after the parallel operation of the two power system is established once again.
A second feature of the present invention is that the output of the inverter is gradually increased at the start of the interconnection with the power system source. When the engine is interconnected with a large load such as a rated load after warming up, it may cause hunting or slowdown (stalling) of the revolution. According to the second feature, while the engine runs steadily, its adverse effects on the system source at the interconnection can be minimized. As a result, the direct current voltage which is susceptible to the exertion of large loads on the engine can be maintained stable and its change which results in misjudgment for the fault detection can be eliminated.
A third feature of the present invention is comprising a means where a generator fault signal is outputted, when the canceling and the re-start of the interconnection with the power system is repeated, the fault detecting means examines whether or not the generator has a fault. According to the third feature, the generator fault signal is outputted when frequent canceling and re-start interconnection with the power system occurred. This prudent judgment improves the accuracy of the fault detection.